Most of the currently available electronic devices, such as the notebook computers, are quickly developed to have high performance, high frequency, high operating speed and highly slim configuration. However, electronic devices with these features tend to produce more and more heat during operation thereof and are therefore subjected to unstable operation and lowered reliability in quality. To overcome the high amount of heat produced during the operation thereof, the electronic devices usually include a centrifugal fan as a heat dissipation device to achieve forced heat dissipation effect. However, the air volume produced by one single centrifugal fan has only limited effect in terms of forced heat dissipation effect. Under this circumstance, a dual centrifugal fan structure has been developed in an attempt to solve the heat dissipation problem in the electronic devices having very limited internal space.
FIGS. 1A, 1B and 1C show a conventional dual centrifugal fan structure 1, which includes a larger fan 10 and a smaller fan 12 located on a top of the larger fan 10. The larger fan 10 has a first air inlet 101, a first air outlet 102 and a first fan wheel 103. The first air inlet 101 is arranged on a bottom of the larger fan 10, and a larger receiving space 105 is defined in the larger fan 10 between the first air inlet 101 and the first air outlet 102 for accommodating the first fan wheel 103. The larger fan 10 is internally provided at a center of the top with a first bearing cup 107 and a first silicon steel plate assembly 108 is fitted around the first bearing cup 107, such that the first fan wheel 103 is rotatably connected to the first bearing cup 107 while encloses the first silicon steel plate assembly 108 therein.
On the other hand, the smaller fan 12 is provided on an outer side with a plurality of spaced lugs 121. The lugs 121 respectively have a through hole 1211 formed thereon and are located corresponding to a plurality of locking holes 109 formed on the top of the larger fan 10. By extending fastening elements 14, such as screws, through the through holes 1211 and the locking holes 109, the smaller fan 12 is fixedly connected to the top of the larger fan 10. The smaller fan 12 has a second air inlet 122, a second air outlet 123 and a second fan wheel 124. The second air inlet 122 is arranged on a top of the smaller fan 12, and the second air outlet 123 defines an air out direction opposite to that of the first air outlet 102. A smaller receiving space 126 is defined in the smaller fan 12 between the second air inlet 122 and the second air outlet 123 for accommodating the second fan wheel 124 therein.
The smaller fan 12 is provided on a center of a bottom thereof with a second bearing cup 127, and a second silicon steel plate assembly 128 is fitted around the second bearing cup 127. The second fan wheel 124 is rotatably connected to the second bearing cup 127 while encloses the second silicon steel plate assembly 128 therein. When the first and the second air outlet 102, 123 of the larger and the smaller fan 10, 12, respectively, guide airflows toward heat-producing elements in the electronic device, such as a notebook computer, the dual centrifugal fan structure 1 can produce increased total air volume to achieve forced heat dissipation effect.
While the conventional dual centrifugal fan structure 1 can produce increased total air volume, the actual amount of the air volume increased is limited. Further, the conventional dual centrifugal fan structure 1 causes another problem. That is, the first fan wheel 103 of the larger fan 10 has blades extended in directions different from those of the blades on the second fan wheel 124 of the smaller fan 12, and part of the airflows guided out of the second air outlet 123 of the smaller fan 12 and out of the first air outlet 102 of the larger fan 10 would interfere with or collide with each other around the larger and the smaller fan 10, 12 to produce eddies, as shown in FIG. 1C. Therefore, the flow field around the dual centrifugal fan structure 1 is unsmooth to adversely affect the overall performance of the dual centrifugal fan structure 1.
Moreover, in manufacturing and assembling the dual centrifugal fan structure 1, increased manufacturing and assembling costs are required because the smaller fan 12 is locked to the top of the larger fan 10 with a plurality of fastening elements 14 to consume extra labor and time. Further, the larger and the smaller fan 10, 12 are two independent centrifugal fans, they respectively include a circuit board for controlling the fan wheel thereof, i.e. the first fan wheel 103 and the second fan wheel 124 are separately controlled via two independent circuit boards. As a result, the fan wheels of the larger and the smaller fan 10, 12 are not consistent in their rotating speeds and require two different power supplies to consume more power and further increase the use cost of the dual centrifugal fan structure 1.
In brief, the conventional dual centrifugal fan structure has the following disadvantages: (1) unsmooth flow field that prevents the fan structure from having improved total performance; (2) increased assembling labor and time costs and power consumption; and (3) increased manufacturing and use costs.
It is therefore desirable to work out a way for overcoming the problems and disadvantages in the conventional dual centrifugal fan structure.